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1. Field of the Invention
This invention relates to floating offshore oil and gas drilling and production equipment in general, and in particular, to a floating cellular hull for a spar-type, deep water, offshore oil and gas drilling and production platform.
2. Description of Related Art
Offshore oil and gas drilling and production operations involve the provision of a vessel, or platform, sometimes called a xe2x80x9crig,xe2x80x9d on which the drilling, production and storage equipment, together with the living quarters of the personnel manning the platform, if any, are mounted. In general, offshore platforms fall into one of two groups, viz., xe2x80x9cfixedxe2x80x9d and xe2x80x9cfloatingxe2x80x9d platforms.
Fixed platforms comprise an equipment deck supported by legs that are seated directly or indirectly on the sea floor. While relatively stable, they are typically limited to relatively shallow waters, i.e., depths of about 500 feet (150 m), although one so-called xe2x80x9ccompliant piled towerxe2x80x9d (xe2x80x9cCPTxe2x80x9d) platform built for the Amerada Hess Corporation, called the xe2x80x9cBaldpatexe2x80x9d tower, is said to be operating at a depth of 1648 ft. (500 m).
Floating platforms are typically employed in water depths of 500 ft. and deeper, and are held in position over the well site by mooring lines anchored to the seabed, or motorized thrusters located on the side of the platform, or both. Although floating platforms are more complex to operate because of their greater movement in response to wind and water conditions, they are capable of operating at substantially greater depths than fixed platforms, and are also more mobile, and hence, easier to move to other well sites. There are several different types of floating platforms, including so-called xe2x80x9cdrill ships,xe2x80x9d tension-leg platforms (xe2x80x9cTLPsxe2x80x9d), semi-submersibles, and xe2x80x9csparxe2x80x9d platforms.
Spar platforms comprise long, slender, buoyant hulls that give them the appearance of a column or spar when floating in their upright operating position, in which an upper portion extends above the waterline and a lower portion is submerged below it. Because of their relatively slender, elongated shape, they present a much smaller area of resistance to wind and wave forces than do other types of floating platforms, and accordingly, have been a relatively successful design over the years. Examples of spar-type floating platforms used for oil and gas exploration, drilling, production, storage, and gas flaring operations may be found in the patent literature in, e.g., U.S. Pat. No. 6,213,045 to S. Gaber; U.S. Pat. No. 5,443,330 to R. Copple; U.S. Pat. Nos. 5,197,826; 4,740,109 to E. Horton; U.S. Pat. No. 4,702,321 to E. Horton; U.S. Pat. No. 4,630,968 to H. Berthet et al.; U.S. Pat. 4,234,270 to T. Gjerde, et al.; U.S. Pat. No. 3,510,892 to G. Monnereau et al.; and U.S. Pat. No. 3,360,810 to B. Busking.
Despite their relative success, spar-type platforms include some aspects that require improvement. For example, because of their elongated, slender shape, they can be relatively more complex to manage during operation than other types of platforms in terms of control over their storage capability, buoyancy, trim, and stability.
Other difficulties relate to their manufacturability. Current manufacturing techniques involve fabricating short cylindrical segments of the hull, stacking the segments successively in a building berth, and joining successive segments to the stack until the full height of the structure is reached. The upright hull structure is then tilted down and skidded onto a barge or a heavy lift vessel for transportation to the well site, where the equipment deck is attached. This construction method has a number of drawbacks. For example, the large diameter cylindrical segments require close alignment to ensure good welds at the segment joints. Accordingly, a substantial number of the segments may be misaligned with each other. Further, a substantial portion of the assembly must be performed at relatively large heights above the ground. Additionally, the assembly berth must be capable of supporting the entire weight of the hull within a relatively small area, and the finished structure must be tilted down before transport.
In light of the foregoing problems, a long-felt, yet unsatisfied need exists in the industry for a floating hull for a spar-type offshore platform that affords a substantially greater flexibility in, and control over, the vessel""s storage capability, buoyancy, trim, and stability, as well as for simpler, more reliable, and less costly methods of making it.
In accordance with one aspect of the present invention, a floating hull of a spar-type platform is provided for supporting an equipment deck used in deepwater offshore oil and gas drilling and production operations that affords a substantially greater flexibility in, and control over, the vessel""s storage capability, buoyancy, trim, stability, and hence, safety, than the floating platforms of the prior art. This is achieved in substantial part by incorporating a plurality of elongated, parallel tubular cells into the hull, positioning some of the cells higher or lower in the water than the other cells, and subdividing the cells into compartments whose buoyancy and trim can be selectably adjusted by the use of fixed or variable ballast, or a combination thereof, e.g., a solid ballast supported in or on the exterior of the cells, and/or a liquid ballast, e.g., petroleum or seawater, selectably pumped into or out of selected ones of the cells or compartments thereof, or a combination of the foregoing types of ballasts.
In one exemplary embodiment, the novel floating hull comprises a tubular central cell that may define a center well, and at least one tubular secondary cell disposed parallel and connected to the central cell with an elongated web. In a variant thereof, the central cell may be connected to the secondary cell by a second elongated web to form a third tubular xe2x80x9cinterstitialxe2x80x9d cell parallel and adjacent to the central and secondary cells. In yet another possible variant, a second tubular secondary cell may be connected to the central cell by a second elongated web, and a third elongated web can connect the first secondary cell to the second secondary cell, thereby forming a third tubular interstitial cell parallel and adjacent to the central and secondary cells. In this manner, a floating hull can be constructed containing a large number of such parallel tubular cells, each having a wide variety of possible cross-sectional shapes, e.g., circular, polygonal, or egg-shaped.
In another exemplary embodiment, the cells of the hull may be formed of a plurality of elongated wall segments, some of which comprise recurvate elements, each having a first end joined to a side wall of the central cell or a first adjacent secondary cell, and an opposite second end joined to the side wall of a second adjacent secondary cell. Alternatively, the elongated wall segments of the cells may comprise webbed elements, each comprising at least one elongated web and at least one elongated flange disposed perpendicular to the web, in the manner of an I-beam. These webbed elements may have cross sections that are T-shaped, I-shaped or II-shaped. The walls of the cells may comprise a metal, e.g., plate steel, reinforced concrete, or a composite material that includes a resin and a reinforcing fiber, e.g., fiberglass.
In another exemplary embodiment, a lower portion of one or more of the cells may extend below the other cells when the hull is floating upright in water, and ballast, either fixed or variable, e.g., a solid ballast or sea water, or both, can be disposed on or in the extended lower portion. The fixed ballast serves to lower the center of gravity of the platform substantially below its center of buoyancy, thereby enhancing the stability of the platform by increasing its natural period above that of the waves in, e.g., a storm condition, and the variable ballast can be used to correct trim and compensate for variations in the load weight of the platform.
In another exemplary embodiment, an upper end of one or more of the cells of the hull can be disposed below the upper ends of the other cells when the hull is floating upright in water, and further, can be positioned to lie either above or below the surface of the water, for trim and stability purposes. Thus, when the upper ends of these cells are positioned below the surface of the water, the hull""s water plane area is decreased, thereby increasing its natural period, whereas, when they are positioned above the surface of the water but below the deck, they minimize wave loads on the hull.
In another exemplary embodiment, one or more longitudinal recesses may be formed in an exterior peripheral surface of the platform, e.g., at the juncture of two cells, and mooring lines and piping may be routed in the recesses to reduce drag on the platform and undesirable, vortex-induced vibrations.
In another exemplary embodiment, a side wall of one or more of the cells includes one or more openings for admitting seawater into and discharging it from the cell or the buoyant compartment contained therein. The buoyant compartments, can comprise one or more horizontal bulkheads disposed within the cells. A pump may be connected to the buoyant compartments and operative to selectably pump air or water into or out of selected ones of the compartments.
In yet another exemplary embodiment, helical strakes can be disposed on an outer peripheral surface of all or some of the cells of the hull to reduce vortex-induced vibrations resulting from currents acting on the platform.
In another aspect of the invention, methods are provided for the efficient construction of the floating hull of the invention. In one exemplary embodiment, the method comprises providing a central tubular cell and a secondary tubular cell disposed parallel to the central cell, and connecting the central cell to the secondary cell with an elongated web, e.g., by a welding or chemical bonding process. Additionally, the central cell may be connected to the secondary cell with a second elongated web such that a third tubular cell is formed parallel and adjacent to the central and secondary cells. Alternatively, a third tubular cell may be provided and arranged parallel to the first and second cells, and then connected to each of the central and secondary cells with respective second and third elongated webs, such that a fourth tubular interstitial cell is formed parallel and adjacent to the central and secondary cells. Using this technique, a cellular floating hull can be built-up quickly and efficiently.
In other exemplary embodiments of the method, the top and bottom ends of the central cell can be closed off, e.g., with bulkheads, thereby rendering it buoyant, and then floating the central cell in a body of water, such as at a graving dock or shipyard, such that a long axis of the cell is disposed horizontally, and the weight of the cell is at least partially borne by the water. This embodiment enables the central cell to be rotated easily in the water about its long axis, e.g., with cranes, before successively connecting one or more secondary cells to it.
A better understanding of the above and many other features and advantages of the present invention may be obtained from a consideration of the detailed description thereof below, particularly if such consideration is made in conjunction with the figures of the appended drawings.